1. Field of the Invention
The present invention relates to a lithographic projection apparatus and device manufacturing methods.
2. Description of the Related Art
The term “programmable patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The terms “light valve” and “spatial light modulator” (SLM) can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
An imaging apparatus as described above is currently employed to make mask writing machines, e.g. by the firm Micronic in Sweden. Such a mask can be used in a lithographic projection apparatus, which repetitively images the mask pattern onto a photosensitive substrate, such as a photoresist-coated silicon wafer, as part of the broader manufacturing process involved in producing integrated devices, such as IC's. The substrate in such a mask writing machine is, for example, a metallized plate (e.g. a Cr-coated quartz or CaF2 plate) that has been coated with a layer of photoresist.
The idea behind such a mask writing machine is that an electronic file of a highly complex mask pattern is used to matrix-address the patterning device, which then diverts a patterned radiation beam onto a small portion of the mask plate. By changing the pattern in the patterned beam in accordance with the electronic file, and concurrently moving the beam over the whole surface of the mask plate, in either a scanning or a stepping motion, the final mask pattern is built up as a sum of combined, juxtaposed sub-patterns from the patterned beam. For this reason, such a machine is sometimes referred to as a “direct-write” machine.
Although machines as described in the previous paragraph have heretofore been used only in the manufacture of masks, it is, at least in principle, possible to use them in the manufacture of semiconductors and other integrated devices. In such a case, the mask plate would be replaced by, for example, a silicon wafer, and the pattern built up on the wafer by the patterning device would correspond to an array of die patterns. However, a major drawback of such an application would be its very low throughput. Whereas current direct-write machines might be expected to achieve a throughput of the order of one substrate per day, a state-of-the-art lithographic projection apparatus has a throughput of the order of one hundred substrates per hour. Nevertheless, it might still be interesting to pursue such an application. For example, in the case of a foundry making a small batch of a particular integrated device (such as a dedicated ASIC), it might be more attractive to endure a slow direct-write process as delivered by a machine as described above rather than to entail the very high costs, often of the order of $50,000-$100,000, of making a special mask for the batch in question. At the moment, such a choice might only be attractive in the case of a very small batch of a very expensive device. However, it would become much more attractive if the throughput of direct-write machines could be increased. More information with regard to conventional lithographic apparatus as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792.
In direct-write machines such as the hereabove referred to, it is desirable not only to produce patterns having black and white shades (binary patterns), but also to allow the creation of intermediate gray shades (grayscaling). In prior art machines, such grayscaling can be achieved in different manners. For example, in a programmable mirror array (PMA) employing tiltable mirrors, each mirror (pixel) can be embodied to allow intermediate tilts (either continuously or discretely) between the zero-tilt and full-tilt extremes. Similarly, in a PMA employing mirrors that translate up and down in a piston-like fashion, causing phase shifts in coherent light reflected from those mirrors, each mirror can be embodied to allow intermediate translations short of the full amplitude, thus allowing phase-shifts at values between 0 and π. However, a disadvantage of these prior-art methods is that they require relatively complicated actuators to move each mirror. As a result, they tend to be relatively slow in their operation, which entails a throughput penalty. Moreover, the manufacture of such actuators is relatively difficult, and can often only be done with a relatively low yield, thus increasing manufacturing costs.